Growth and differentiation factor-8 (GDF8), also known as myostatin, is a secreted protein and member of the transforming growth factor-beta (TGF-β) superfamily of structurally related growth factors. Members of this superfamily possess growth-regulatory and morphogenetic properties (Kingsley et al. (1994) Genes Dev. 8:133-46; Hoodless et al. (1998) Curr. Topics Microbiol. Immunol. 228:235-72). Human GDF8 is synthesized as a 375 amino acid precursor protein that forms a homodimer complex. During processing, the amino-terminal propeptide, known as the “latency-associated peptide” (LAP), is cleaved and may remain noncovalently bound to the homodimer, forming an inactive complex designated the “small latent complex” (Miyazono et al. (1988) J. Biol. Chem. 263:6407-15; Wakefield et al. (1988) J. Biol. Chem. 263:7646-54; Brown et al. (1999) Growth Factors 3:35-43; Thies et al. (2001) Growth Factors 18:251-59; Gentry et al. (1990) Biochemistry 29: 6851-57; Derynck et al. (1995) Nature 316:701-05; Massague (1990) Ann. Rev. Cell Biol. 12:597-641). Proteins such as follistatin and follistatin-related proteins including GASP-1 (Gamer et. al. (1999) Dev Biol. 208:222-232, US Patent Pub No. 2003-0180306-A1; US Patent Pub No. 2003-0162714-A1) and bind mature GDF8 homodimers and inhibit GDF8 biological activity.
An alignment of the deduced GDF8 amino acid sequence from various species demonstrates that GDF8 is highly conserved (McPherron et al. (1997) Proc. Natl. Acad. Sci. U.S.A. 94:12457-61). The sequences of human, mouse, rat, porcine, and chicken GDF8 are 100% identical in the C-terminal region, while baboon, bovine, and ovine GDF8 differ by a mere 3 amino acids at the C-terminus. The high degree of GDF8 conservation across species suggests that GDF8 has an essential physiological function.
GDF8 has been shown to play a major role in the regulation of muscle development and homeostasis by inhibiting both proliferation and differentiation of myoblasts and satellite cells (Lee and McPherron (1999) Curr. Opin. Genet. Dev. 9:604-7; McCroskery et al. (2003) J. Cell. Biol. 162:1135-47). It is expressed early in developing skeletal muscle, and continues to be expressed in adult skeletal muscle, preferentially in fast twitch types. GDF8 has also been implicated in the production of muscle-specific enzymes (e.g., creatine kinase) and myoblast proliferation (WO 00/43781).
Overexpression of GDF8 in adult mice results in significant muscle loss (Zimmers et al. (2002) Science 296:1486-88). Similarly, various studies indicate that increased GDF8 expression is associated with HIV-induced muscle wasting (Gonzalez-Cadavid et al. (1998) Proc. Natl. Acad. Sci. U.S.A. 95:14938-43). In contrast, GDF8 knockout transgenic mice are characterized by a marked hypertrophy and hyperplasia of the skeletal muscle and altered cortical bone structure (McPherron et al. (1997) Nature 387:83-90; Hamrick et al. (2000) Bone 27:343-49). Also, natural mutations that render the GDF8 gene inactive have been shown to cause both hypertrophy and hyperplasia in both animals and humans (Lee and McPherron (1997), supra). For example, increases in skeletal muscle mass are evident in natural GDF8 mutations in cattle (Ashmore et al. (1974) Growth 38:501-07; Swatland et al. (1994) J. Anim. Sci. 38:752-57; McPherron et al., supra; Kambadur et al. (1997) Genome Res. 7:910-15).
A number of human and animal muscle and bone disorders are associated with functionally impaired muscle tissue, and thus, may also be associated with GDF8. For example, GDF8 may be involved in the pathogenesis of amyotrophic lateral sclerosis (“ALS”), muscular dystrophy (“MD”; including Duchenne's muscular dystrophy, fascioscapular muscular dystrophy, and facioscapulohumeral muscular dystrophy), muscle atrophy, carpal tunnel syndrome, organ atrophy, frailty, congestive obstructive pulmonary disease (COPD), sarcopenia, cachexia, and muscle wasting syndromes caused by other diseases and conditions.
GDF8 is also believed to participate in numerous other physiological processes and related disorders, including glucose homeostasis during type 2 diabetes development, impaired glucose tolerance, metabolic syndromes (i.e., syndromes (e.g., syndrome X) involving the simultaneous occurrence of a group of health conditions (which may include insulin resistance, abdominal obesity, dyslipidemia, hypertension, chronic inflammation, a prothrombotic state, etc.) that places a person at high risk for type 2 diabetes and/or heart disease), insulin resistance (e.g., resistance induced by trauma such as burns or nitrogen imbalance), and adipose tissue disorders (e.g., obesity, dyslipidemia, nonalcoholic fatty liver disease, etc.) (Kim et al. (2000) Biochem. Biophys. Res. Comm. 281:902-06). Currently, few reliable or effective therapies exist to treat these disorders. The pathology of these processes indicates GDF8 as a potential target in the treatment of these related disorders.
In addition to neuromuscular disorders in humans, there are also growth factor-related conditions associated with a loss of bone, such as osteoporosis and osteoarthritis, which predominantly affect the elderly and/or postmenopausal women. Such metabolic bone diseases and disorders include low bone mass due to chronic glucocorticoid therapy, premature gonadal failure, androgen suppression, vitamin D deficiency, secondary hyperparathyroidism, nutritional deficiencies, and anorexia nervosa. Although many current therapies for these conditions function by inhibiting bone resorption, a therapy that promotes bone formation would be a useful alternative treatment. Because GDF8 plays a role in bone development as well as muscular development, GDF8 is also an excellent pharmacological target for the treatment of bone-degenerative disorders.
Like other members of the transforming growth factor-β (TGF-β) family, GDF8 is synthesized as a 376 amino acid precursor protein containing a signal sequence, a N-terminal propeptide domain, and a C-terminal domain considered as the acitve molecule. GDF8 is secreted in a latent form by binding to it's propeptide (latency-associated peptide, LAP); proteolytic processing between the propeptide domain and the C-terminal domain produces an N-terminal propeptide and the mature form of GDF8. Both unprocessed and mature GDF8 form disulfide-linked dimers, and the processed GDF8 dimer represents the only active form of the protein. In serum, as well as in skeletal muscle, GDF8 can be found bound to several proteins that are able to modulate its activation, secretion or receptor binding.
GDF8 exerts its effects through a transmembrane serine/threonine kinase heterotetramer receptor family, activation of which enhances receptor transphosphorylation, leading to the stimulation of serine/threonin kinase activity. It has been shown that the GDF8 pathway involves an active GDF8 dimer binding to the high affinity receptor, ActIIRB, which then recruits and activates the transphosphorylation of the low affinity receptor, ALK4/ALK5. It has also been shown that the proteins Smad 2 and Smad 3 are subsequently activated and form complexes with Smad 4 and are then translocated to the nucleus, which then activate target gene transcription. Lee and McPherron (Proc Natl Acad Sci USA 2001, 98:9306-9311) have demonstrated that the ActRIIB receptor was able to mediate the influence of GDF8 in vivo, as expression of a dominant negative form of ActIIRB in mice that mimics GDF8 gene knockout.
It has been shown that under the influence of GDF8, C2C12 myoblasts accumulate in the G0/G1 and G2 phases of the cell-cycle, consequently decreasing the number of S-phase cells. Also, GDF8 induces failure of myoblast differentiation, associated with a strong decrease in the expression of differentiation markers. GDF8 expression also decreases the apoptotic rate of cells under both proliferation and differentiation conditions (Thomas et al., J. Biol Chem 2000, 275:40235-40243).
Inhibition of myostatin (GDF8) expression leads to both muscle hypertrophy and hyperplasia (Lee and McPherron, supra; McPherron et al., supra). Myostatin negatively regulates muscle regeneration after injury, and lack of myostatin in GDF8 null mice results in accelerated muscle regeneration (McCroskery et al., (2005) J. Cell. Sci. 118:3531-41). Human anti-GDF8 antibodies (U.S. Published Application No. 2004/0142382) have been shown to bind GDF8 and inhibit GDF8 activity in vitro and in vivo, including GDF8 activity associated with negative regulation of skeletal muscle mass and bone density. For example, myostatin-neutralizing antibodies increase body weight, skeletal muscle mass, and muscle size and strength in the skeletal muscle of wild type mice (Whittemore et al. (2003) Biochem. Biophys. Res. Commun. 300:965-71) and the mdx mouse, a model for muscular dystrophy (Bogdanovich et al. (2002) Nature 420:418-21; Wagner et al. (2002) Ann. Neurol. 52:832-36). Furthermore, myostatin antibodies in these mice decrease the damage to the diaphragm, a muscle that is also targeted during ALS pathogenesis. It has been hypothesized that the action of growth factors, such as HGF, on muscle may be due to inhibition of myostatin expression (McCroskery et al. (2005), supra), thereby helping to shift the balance between regeneration and degeneration in a positive direction. However, these prior art antibodies were not specific for GDF8, i.e., these antibodies have high affinity for other members of the TGF-β superfamily, such as BMP11.
To date, all known inhibitors of GDF8 activity (e.g., propeptide, soluble ActRIIB receptor, anti-GDF8 antibodies, etc.) also neutralize the biological activities of other factors (e.g., BMP11, activin, etc.) that have important biological functions. For example, activin and BMP11 play important roles during embryogenesis. Activin βA is identified as a critical gonadal growth factor, and BMP11 is responsible for homeotic transformation of the axial skeleton. Homozygous BMP11 knockout mice are perinatal lethal; mice with one wild type copy of the BMP11 gene are viable but have skeletal defects. Since activin and BMP11 play important roles during embryogenesis, an antagonist that inhibits GDF8 and other factors, e.g., BMP11 poses theoretical safety risks that could present either as toxicity in treated patients or as reproductive toxicity in, e.g., women of childbearing potential. Thus, there is a need for compounds and methods of treatment that contribute to an overall increase in muscle mass and/or strength and/or bone density, particularly in humans, but do not interfere with, e.g., BMP11. In other words, there is a need for specific inhibition of GDF8 activity in treatments of GDF8-associated disorders for which it is desirable to increase muscle mass, size, strength, etc., particularly in women with childbearing potential.
As methods of using GDF-8 modulating agents are developed, there is a need to develop methods to monitor and to optimize the administration of such agents to an individual. In particular, the ability to measure GDF-8 protein levels in biological fluids has important implications for ongoing clinical trials. For example, circulating GDF-8 levels might be diagnostic for pathological conditions that could benefit from anti-GDF-8 therapy, or might predict which individuals are more likely to respond to anti-GDF-8 therapy. In addition, changes in GDF-8 levels in peripheral blood during anti-GDF-8 treatment may be an early indicator of later measurable response in muscle mass and/or function.
In order to accomplish such optimization goals, methods to detect or monitor GDF-8 protein levels in biological fluids, such as serum and plasma are needed. It is desirable to monitor GDF-8 levels prior to, during, and post treatment with a GDF-8 modulating agent in order to identify appropriate individuals for such treatment, monitor responses to the treatment, and follow post-treatment progress, for example. In particular, methods allowing the detection and/or quantitation of endogenous GDF-8 levels in response to administration of GDF-8 modulating agents, including GDF-8 inhibitors and anti-GDF-8 antibodies are needed.
It is accordingly a primary object of the present invention to provide compounds and methods that specifically inhibit GDF8 activity as well as immunological assays to detect and quantitate GDF-8 levels in biological samples, such as, for example, in serum and plasma.